Does the Bible say weed is a sin?

There is no obvious reference to marijuana in the Bible. However there is quite a bit about misuse of alcohol. For example:

Pro 23:30-35 Those who tarry long over wine; those who go to try mixed wine. (31) Do not look at wine when it is red, when it sparkles in the cup and goes down smoothly. (32) In the end it bites like a serpent and stings like an adder. (33) Your eyes will see strange things, and your heart utter perverse things. (34) You will be like one who lies down in the midst of the sea, like one who lies on the top of a mast. (35) “They struck me,” you will say, “but I was not hurt; they beat me, but I did not feel it. When shall I awake? I must have another drink.”

Marijuana is not the same as alcohol, of course. The scientific evidence for the harmful physical effects of marijuana is less clear than for tobacco and alcohol, though the evidence for some harmful mental effects is sound. It may be less harmful than alcohol and tobacco. Putting it another way, tobacco and alcohol may be even more harmful than marijuana. That is hardly a recommendation for marijuana, but a warning against use of tobacco and excessive drinking.

In nearly all places in the world marijuana use is not legal. So even if its use could be proved to be safe it falls under the command to submit to authority:

Rom 13:1-6 Let every person be subject to the governing authorities. For there is no authority except from God, and those that exist have been instituted by God. (2) Therefore whoever resists the authorities resists what God has appointed, and those who resist will incur judgment. (3) For rulers are not a terror to good conduct, but to bad. Would you have no fear of the one who is in authority? Then do what is good, and you will receive his approval, (4) for he is God’s servant for your good. But if you do wrong, be afraid, for he does not bear the sword in vain. For he is the servant of God, an avenger who carries out God’s wrath on the wrongdoer. (5) Therefore one must be in subjection, not only to avoid God’s wrath but also for the sake of conscience. (6) For because of this you also pay taxes, for the authorities are ministers of God, attending to this very thing.